Time division multiplexing/time division multiple access (TDM/TDMA) repeater systems are known. Such systems are characterized by an inbound channel, utilized by a multiplicity of users, each of which being permitted the use of one or more non-overlapping time periods (slots) in which to transmit information. A continuous outbound channel exists, also time slotted to facilitate segregation of information directed to the various users, wherein at least some of the outbound transmitted information is derived from the received inbound information.
The use of the aforementioned TDM/TDMA approach to provide widearea coverage trunking radio systems is also known. The requirements for these systems are great. On the one hand, it is desired to provide the variety of system configurations potentially necessary to provide wide area networked coverage, including satellite receivers, simulcast, etc. This may involve compensating for considerable delay in the system infrastructure. On the other hand, it is desired to maintain all the features of TDM including multi-radio capabilities such as full duplex, voice plus data, conferencing, priority monitor, emergency preemption, etc. Furthermore, system capacity and performance should not be compromised.
A key problem is that of minimizing the bulk audio delay through the entire system. Excess delay introduces a confusion factor and reduces communications efficiency and user acceptance. The use of TDM already may introduce a substantial fixed amount of delay due to a relatively large RF channel framing such as, for example, 250 msec one-way. Thus, it is particularly important to minimize any additional delay introduced by any scheme for networking sites.
The basic goals of a repeater in any wide area networked radio trunking system are as follows: For any transmitter node, it is desired to repeat a user's information that is somehow obtained from one or more signals either received at that node or received via alternate communications paths from potentially numerous other remote nodes throughout the network. The added audio delay should be as small as possible. In a multiple user TDM/TDMA system, there should also be no impact on the features or capabilities of the basic single-site TDM system. A key advantage of certain TDM/TDMA systems, however, is that a subscriber can transmit during one time slot and have all the other time slots available for reception to provide features such as receiving simultaneous data, monitoring the control channel (slot) for various purposes, conferencing, full duplex, and so forth. This requirement precludes the approach of shifting the outbound repeater timing relative to the inbound timing, since an overlap zone is formed that prevents subscriber reception of all the other outbound slots while transmitting, thereby limiting some of the TDM system features. Thus, it is desirable that the transmitter output TDM framing from all the nodes should be such that one user's information is repeated essentially coincident with when that same user is being received. Means for combining multiple signals from satellite receivers for retransmission is well known in the art. Likewise, introduction of infrastructure delay to align the timing of signals for simulcast retransmission is also well known.
In the past, several schemes have been used to achieve this desired synchronization.
One scheme has introduced additional guard time at the end of each inbound slot. The outbound slots are synchronized to the inbound frame and the received information is repeated slightly delayed in time within the same frame time slot. The impact of having to repeat this frame at a remote node is that the guard time must be increased to allow for the maximum backbone delay incurred from any receiver site to any transmitter in the network. The obvious disadvantage of this approach is that it would require such a substantial increase in propagation allowance that the channel capacity would be greatly diminished.
A second approach likewise provides coincident inbound and outbound frames. Information received during a first slot is delayed for retransmission during a later slot. The main problem with this approach is that the additional incurred delay, over and above the basic TDM/speech coding delay, can be quite large. At least one full time slot's information, representing a frames worth of speech, is required to be received even if the next adjacent slot is used to repeat the inbound slot.
As a result, there is a need for an improved method for repeating TDM/TDMA frames in a trunked wide area network environment.